1. Field of the Invention
The present invention generally relates to integrated circuit devices using copper wiring technology and more particularly to reducing failures at the interface between the last wiring level copper contact pad and the controlled collapse chip connect (C4) structure.
2. Background Description
Copper back end of line (BEOL) wiring technologies offer substantial performance and potential cost advantages over the traditional aluminum technologies used for integrated circuit device fabrication. However, copper metalization is inherently more process-sensitive than aluminum, and less compatible with established chip/packaging interconnection technology. Copper metal readily forms a native oxide, which is not self-passivating like that of aluminum, but rather can continue to react in the presence of air and moisture to form corrosion products. Additionally, some inorganic dielectric materials, in particular silicon nitride, which are used as a barrier layer to prevent copper diffusion into silicon dioxide, do not adhere well to copper.
Initial C4/packaging qualification work on CMOS technology produced a significant number of fails related to the delamination of the nitride barrier layer from the copper pad during chip-pull C4 testing. The preferred fail mode during these tests is for the C4 Pb/Sn ball to fail cohesively, in the manner of a xe2x80x9ctaffy pullxe2x80x9d.
As initially defined, CMOS technologies made use of a last-metal (LM) level copper pad overcoated with silicon nitride, the silicon nitride containing an open via (window) fully landed on the copper pad, as the contact point for ball limiting metallurgy (BLM)/C4 or wirebonding to a package. With this technology, it was observed that pull-testing of C4s produced significant levels of fails characterized not by cohesive C4 fail within the lead tin, but rather by separation at the copper interface with the passivating nitride barrier layer. This type of fail is catastrophic, often originating at the copper/nitride interface located immediately adjacent to the BLM pad edge, and pulling back across the entire pad and beyond, occasionally disrupting adjacent wiring line structures as well.
It is therefore an object of the present invention to provide improved adhesion of the C4 structure on copper by operating on the critical interface without introduction of critical photo layers, or increase in wiring line sheet resistance.
The disclosed structure entirely eliminates the critical copper/nitride interface that has been shown to fail during chip pull, while creating large areas of films and interfaces that are inherently strong. The structure does not increase wiring line electrical resistance (Rs) like copper silicide and it does not involve critical mask levels like full cladding options.
A typical structure for the final wiring level and its associated passivation overlayers consists of a last metal (LM) copper pad, covered with a tri-layer final passivation film stack comprised of an approximately 700 xc3x85 thick barrier layer immediately above the copper pad., an approximately 4500 xc3x85 thick PECVD oxide layer finished with an approximately 4500 xc3x85 thick PECVD nitride layer. A via window is imaged in the nitride/oxide/nitride layer and the window is fully contained on the underlayer copper pad. A polyimide layer optionally may be applied over the nitride/oxide/nitride to provide reduction of mechanical stress on the integrated circuit die induced by the packaging compound. If a polyimide layer is used, a via window is imaged concentric with the via window in the nitride/oxide/nitride.
Various attempts to improve nitride to copper adhesion so as to produce the desired failure mode during C4 (pull) test have proven to be marginally successful, have undesired attributes or are exceptionally difficult to manufacture. These attempts include forming oxide pillars within the bound of the copper pad for improved adhesion through increased nitride/oxide area percentage, formation of copper silicide prior to nitride deposition and cladding of the copper pad or pad plus associated wiring line with Ta or TaN. Oxide pillars reduce BLM to pad contact area and are marginally effective at changing failure mode. Copper silicide changes failure mode to the desired cohesive fail within the PbSn solder ball but also increases the resistance of the copper wiring conductor. Cladding with another material can be effective in changing the C4 failure mode, but introduces additional films deposition and imaging. Imaging in particular can be difficult in tight pitch wiring areas if a subtractive etch process is used, i.e. critical overlay, or in large copper pad areas if a damascene process is used, i.e. polish through. The current invention eliminates the problem nitride to copper interface in the pad area while avoiding challenging manufacturing processes.